I'd recommend Git, a source control program invented by the guy that made Linux. GM has built-in support for it, but the command-line version lets you do more powerful stuff.
Basic flow is:
guy 1 does
git add -A
git commit -m "added new stuff"
guy 2 does
(this won't work until after you've added a remote repo, which is a cloud copy of the repository that both of you can push stuff to / pull changes from - bitbucket and github are the two leading providers on the market)
These videos helped me understand Source Control in GMS2. They also helped me understand the difference between Git and GitHub, taught me two popular options for using Git (GitHub and BitBucket) and two different ways to integrate them into a GMS2 project (through the GUI and through a command line.)